My way to get this done was to add the machines was the following:
Add the Machines the regular way by doing the enlist and commission steps.
They will not touch your installed system but gather information about the machine that MAAS needs to handle them.
Now the dirty part:
On the MAAS region controller run the following command:
DON'T RUN THIS BLINDLY BUT UPDATE THE WHERE CLAUSE:
sudo -u postgres psql
postgres=# \c maasdb
UPDATE maasserver_node SET status='6',netboot='f',bios_boot_method='pxe',osystem='centos',distro_series='centos70',owner_id='3' WHERE hostname != <name of your controller in single quotes>;
Status 6 means Deployed.
It's not very nice but it solved my problem.
As soon as I enable the maas-dhcpd and the systems boot they don't anymore go into enlistment or commissioning.
It is a good idea to deploy another system that is very similar to the ones you want to import.
To get an idea about how other systems records look do a:
SELECT * FROM maasserver_node;
I don't know whether this is the most minimal update command you can run but it solved my problem.
Tested with MAAS 2.6.0 running on Ubuntu 20.04.